Various methods are known for applying solder selectively to printed circuit boards (PCBs) in such a way that following the assembly of the boards with SMD components, the boards can be electrically and mechanically connected to the components by reflow soldering methods.
In particular, the prior art uses a method in which the solder deposit is applied to selected regions of the PCB in the form of a deposit of solder by screen or stencil printing with soldering pastes. While this method is in common use today, nevertheless, in many respects, this method has many problems and cannot be considered optimal. The investment cost of screen printing equipment is considerable; thin solder deposits are often produced; thixotropic properties of pastes change; soldering pastes are expensive; resolution is limited; optoelectronic positional recognition of assembly systems involves great difficulties; the flexibility of interlinked production lines having screen printing is greatly impaired because these machines entail considerable set-up times and adjustments; heating speed of the introduction of heat in reflow soldering with solder pastes has proven to be limited because, in the heating process, volatile ingredients and solvents from the paste have to evaporate which takes time; and, other disadvantages associated with solder paste are: a)bubble formation, b)oxidation of the soldering paste, c) fine granulation and the like. An additional problem, not associated only with screen printing is that the solder forms "bumps", that is, the solder has a more or less convex cross-sectional shape making placement difficult and contributing to rework.
Another previously used method in the prior art is known as immersion application of solder to PCBs. In this method, a prepared PCB is dipped into and removed from a solder bath. After being removed and following a cool down phase, the metallized regions of the PCB provided with a solder resist means are provided further with a solder deposit, which, however, forms the solder bump because of the high surface tension of solder. The height of the solder deposit is also dependent on the dimensions in the plane of the PCB of the regions to which solder is to be applied, so that when such regions have different dimensions, variably high solder deposits necessarily result.
U.S. Pat. No. 5,051,339 issued Sep. 24, 1991 to Friedrich et al., the so-called "OPTIPAD" process, is an attempt to overcome some of these disadvantages, the major one being the solder "bump". Because this is an immersion process, there are no foreign ingredients, that is, no ingredients but solder in the solder deposits.
The process, simply stated, involves procuring a PCB having thereon a solder mask with pads exposed, laminating thereto a photoimageable layer of perhaps 5 mil in thickness, exposing and developing so that everything is masked but the pads to be soldered, immersing the thus prepared PCB into molten solder, and then contacting the board with a closure element to maintain the solder in place and keep the surface flat until it solidifies. Optionally, the limiting layer, a photoimageable solder resist mask or a regionally applied foil layer, can be left on if the component has leads which must be connected to the pads or can be removed if SMD lead-less components are used.
The "OPTIPAD" process requires the use of a 5 mil temporary coating which is imaged, developed where the pads are and then immersed in molten solder. Others are attempting to modify this process by screening the molten solder into the pad wells. Stripping the temporary layer leaves behind a 5 mil high pillar of solder. Aside from the need for expensive equipment which is not commercially available, this process has two major problems. When the temporary coating sees molten solder it cures extensively and is difficult to remove even when stripping with caustic soda which can attack the permanent mask. In addition, when the 5 mil solder mask is stripped it leaves high pillars of solder which in fine pitch applications when mated with their components collapse and generate extensive shorts.
In addition to the '339 patent mentioned above, there are two other relevant publications; a paper by W. J. Maiwald of Siemens entitled, "Reliable Reflow Soldering Techniques using Preformed Solid Solder Deposits, Part 2--The Assembly Process" and the associated paper by M. Weinhold of DuPont entitled, " . . . Part 1--The Printed Circuit Fabrication Process".
The Siemens process combines a few known technologies and produces a flat solid solder deposit on the PCB. The processing steps are applying solder pastes onto boards with permanent solder masks, melting of the solder paste and flattening of the round, humped solder deposits by a thermal/mechanical process.
Neither of these two processes have been completely successful. The former is run in an extremely aggressive environment, that is, molten solder at 450 degrees F. Both require highly specialized equipment, although in the latter attempts are being made to run this process in a conventional multilayer press normally used for PCBs. The problem here is that this type of press requires about 2000 psi before activation, and though the platens see the top of the solder first, they then contact the PCB itself and thermally shock the laminate, oftentimes scorching and mechanically damaging the solder mask as well. Furthermore, the solder when compressed, squeezes sideways as a very thin film or foil. While this problem can be solved by various techniques to remove the foil, it results in extensive and expensive rework and fine droplets of the thus squeezed out solder end up as solder balls which are difficult to remove.
The major problems which both of these processes have attempted to address are the following: opens and shorts, squeezing out of solder and resultant solder bridging; low packing density and inability to solder with high pin counts without extensive design modifications; inability of the paste to maintain its profile after printing; achieving the required shape of the deposit; presence of a "hump", the meniscus, makes positioning of fine pitch components difficult resulting in unacceptable skewing tolerances; solder balls; shelf-life of solder joint; poor solderability due to too thin deposits; poorly defined soldering gap; inability to quantify and standardize solder deposit and solder gap; and yield after soldering. First-pass yields in standard applications are running at 60-70%, while for many fine pitch cases they are only 10% and, therefore, rework is extensive.
The solutions and benefits both of these approaches have attempted to achieve in addition to solving the above problems are as follows: removal of the solder paste printing process from the assembler's operation; the assembler would then use a 100% tested presoldered board since faults arising from solder application can be separately controlled, eliminated or reworked at the PCB fabricator without the obstruction of components; the problem of solder paste deposits being deformed when the component terminals touch down would be non-existent; components can be placed on a flat surface which would permit the use of fine-pitch flat packs and TAB assembly with automated equipment; the possibility of quantifiable and standardized solder deposits and solder joints; better yield after soldering with considerable reduction of rework, higher first pass yields; better overall quality of boards and solder joints, improved product consistency; and, lower cost due to faster throughput in assembly with shorter SMD assembly lines.
In conclusion, both of these new processes leave much to be desired and are not very practical in their present state of development. As a consequence, the study which resulted in this subject invention was undertaken.